Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°).
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of one or more receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
Magnetic resonance angiography (MRA) is a group of techniques based on MR imaging for the purpose of imaging blood vessels. MRA is used to generate images of the arteries in order to evaluate them for stenosis, occlusion or aneurysms. MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs. Further known are magnetic resonance venography (MRV) techniques which are used to generate images of the veins.
MRA methods can be divided into “bright blood” and “dark blood” techniques. In bright blood angiography or time of flight (TOF) angiographs, MR signals from flowing blood are optimized, while MR signals from stationary tissue are suppressed. Dark blood MRA typically uses a short echo time in combination with large flip angles for excitation of magnetic resonance. As flowing blood enters the area actually being imaged it has seen a limited number of RF excitation pulses so it is not saturated. Consequently, the MR signals from the flowing blood are of much higher amplitude than the MR signals from the saturated stationary tissue. The use of large flip angles leads to high MR signal amplitudes from the freshly inflowing moving spins of the blood within the imaged volume. Simultaneously, an effective suppression of the static spins is achieved by the use of large flip angles. As a consequence, bright blood vessels are depicted against a dark background in the resulting bright blood MR images.
In contrast, dark blood MRA methods utilize a flow-related signal void. The MR signals from flowing blood are suppressed, while the MR signals from stationary tissue are optimized. In other words, the flowing blood is made to appear dark or black in the resulting MR image due to an absence or minimum of MR signal emanating from the flowing blood. Dark blood MRA typically employs a small flip angle and a long echo time. Due to the long echo time, the MR signal from moving blood spins has decayed relative to its surroundings at the instant of signal acquisition. A low flip angle is employed for excitation of magnetic resonance in order to maintain MR signals from the static tissue surrounding the blood vessels.
In practice it is sometimes desirable to apply both dark blood and bright blood MRA in order to have the complementary information of the two contrast types available for diagnosis. The problem is that two complete scans are required in order to obtain both bright blood and dark blood MR images which results in a correspondingly long scan time.
From the foregoing it is readily appreciated that there is a need for an improved MR imaging technique. It is consequently an object of the invention to enable dual contrast MR imaging at a reduced acquisition time.